When caller is placed on parking for a while he gets disconnected. (3-5 min)

All the phones has firmware

I have one client with a similar issue, after 8 mins on Hold they get disconnected.

Other than that have you checked how long the parked call should live and then where it should go after the live time (i.e. to voice mail, back to IVR, etc)?

Is it really anywhere from 3-5 mins (something you’ve tested/verified) or is that what people are telling you?

That is what they are saying and you are right it is when you placed callers on hold for a longer period of time
Parking timeout is 3 min and the it goes back to extension